Lubricating Oil Composition and Method of Improving Efficiency of Emissions Control System

ABSTRACT

The disclosed technology relates to a lubricating oil composition comprising at least one base oil and at least one overbased low molecular weight zinc dialkyldithiophosphate derived from one or more dithiophosphoric acids represented by the formula (I) and/or a metal salt thereof, wherein in formula (I), R 1  and R 2  are independently hydrocarbyl groups, the average total number of carbon atoms in R 1  and R 2  in formula (I) being in the range from 4 to 10, the metal ratio of the overbased low molecular weight zinc dialkyldithiophosphate being at least about 1.15:1, the lubricating oil composition being characterized by a phosphorus concentration of up to about 0.12% by weight. The disclosed technology also relates to a method of lubricating an internal combustion engine and improving the efficiency of the emissions control system of the engine.

TECHNICAL FIELD

The disclosed technology relates to a lubricating oil composition and to a method of improving the efficiency of the emissions control system of an internal combustion engine. The lubricating oil composition may be characterized by the inclusion of at least one overbased low molecular weight zinc dialkyldithiophosphate. The method relates to using the lubricating oil composition to lubricate an internal combustion engine equipped with the exhaust gas after treatment device. In one embodiment, the exhaust gas after treatment device comprises a catalyst and the contamination of the catalyst is reduced.

BACKGROUND OF THE INVENTION

For decades phosphorus in the form of zinc dialkyldithiophosphates (ZDDPs) have been used as extreme pressure (EP) and antiwear additives in engine oils. A problem with the use of phosphorus, however, is that it contaminates catalysts used in exhaust gas after treatment devices and thereby reduces their effectiveness. In response to this problem, phosphorus concentration has been reduced for some SAE passenger car engine oil classifications. With the introduction of ILSAC GF-1, phosphorus levels were limited to no more than 1200 parts per million (ppm) and with GF-3 to 1000 ppm. Even at these levels of phosphorus, however, catalyst contamination is still an issue. The problem therefore is to provide adequate engine lubrication and at the same time reduce catalyst contamination. The disclosed technology, in at least one embodiment, provides a solution to this problem.

SUMMARY OF THE INVENTION

The disclosed technology relates to a lubricating oil composition comprising at least one base oil and at least one overbased low molecular weight zinc dialkyldithiophosphate derived from one or more dithiophosphoric acids represented by the formula

and/or metal salt thereof, wherein in formula (I), R¹ and R² are independently hydrocarbyl groups, the average total number of carbon atoms in R¹ and R² in formula (I) being in the range from 4 to 10, the metal ratio for the overbased low molecular weight zinc dialkyldithiophosphate being at least about 1.15:1, the lubricating oil composition being characterized by a phosphorus concentration of up to about 0.12% by weight.

In one embodiment, the disclosed technology relates to a method of lubricating an internal combustion engine and improving the efficiency of the emissions control system of the engine, the emissions control system being equipped with a catalyst containing exhaust gas after treatment device, the method comprising:

(A) lubricating the engine with a lubricating oil composition comprising at least one base oil and at least one overbased low molecular weight zinc dialkyldithiophosphate derived from one or more dithiophosphoric acids represented by the formula

and/or metal salt thereof, wherein in formula (I), R¹ and R² are independently hydrocarbyl groups, the average total number of carbon atoms in R¹ and R² in formula (I) being in the range from 4 to 10, the metal ratio of the overbased low molecular weight zinc dialkyldithiophosphate ratio being at least about 1.15:1, the lubricating oil composition being characterized by a phosphorus concentration of up to about 0.12% by weight;

(B) generating a lean-phosphorus containing exhaust gas; and

(C) contacting the catalyst in the exhaust gas after treatment device with the lean-phosphorus containing exhaust gas.

DETAILED DESCRIPTION

The term “low molecular weight zinc dialkyldithiophosphate” may refer to one or more zinc dialkyldithiophosphates derived from one or more dithiophosphoric acids represented by formula (I) wherein the average total number of carbon atoms in R¹ and R² for the one or more dithiophosphoric acids represented by formula (I) is up to 10, and in one embodiment in the range from 4 to 10.

The term “high molecular weight zinc dialkyldithiophosphate” may refer to one or more zinc dialkyldithiophosphates derived from one or more dithiophosphoric acids represented by formula (I) wherein the average total number of carbon atoms in R¹ and R² in formula (I) is greater than 10.

The term “overbased” is a term of art which is generic to well known classes of metal containing compositions comprising metal salts and/or metal complexes. These compositions may also be referred to as “basic,” “superbased,” “hyperbased,” “high-metal containing salts,” and the like. Overbased metal compositions may be in the form of inert organic liquid solutions characterized by a metal content in excess of that which would be present according to the stoichiometry of the metal (e.g., zinc) and the particular acidic organic compound (e.g., a low molecular weight dialkyldithiophosphoric acid) reacted with the metal. Thus, for example, if a low molecular weight dialkyldithiophosphoric acid is neutralized with a basic metal compound (e.g., zinc oxide), the “neutral” or “normal” metal salt produced will contain one equivalent of zinc for each equivalent of acid. On the other hand, an overbased metal composition will contain more than the stoichiometric amount of the metal. For example, a low molecular weight dialkyldithiophosphoric acid and/or metal (e.g., zinc) salt thereof may be reacted with a zinc base and the resulting overbased low molecular weight zinc dialkyldithiophosphate may contain an amount of zinc in excess of that necessary to neutralize the acid, for example, about 1.15 times as much zinc as present in the neutral salt or a zinc excess of about 0.15 equivalents. The actual stoichiometric excess of zinc may vary considerably, for example, from about 0.10 to about 1.0 equivalents, and in one embodiment from about 0.2 to about 0.5 equivalents.

The term “metal ratio” may be used herein to designate the ratio of the total chemical equivalents of metal (i.e., zinc) in an overbased composition (e.g., overbased low molecular weight zinc dialkyldithiophosphate) to the chemical equivalents of the metal in the corresponding neutral salt. Thus, for example, the metal ratio for a neutral low molecular weight zinc dialkyldithiophosphate is 1:1, and the metal ratio for the overbased low molecular weight zinc dialkyldithiophosphate with a metal excess of 0.15 equivalents discussed above is 1.15:1.

The term “overbased low molecular weight zinc dialkyldithiophosphate derived from one or more dithiophosphoric acids” may refer to an overbased low molecular weight zinc dialkyldithiophosphate which is derived from the dialkyldithiophosphoric acid and/or metal (e.g., zinc) salt of the dialkyldithiophosphoric acid.

The overbasing of the dithiophosphoric acids or their zinc salts with zinc oxide may be accomplished using catalytic amounts of alkali metal hydroxides such as sodium hydroxide or potassium hydroxide as in U.S. Pat. No. 5,015,402, or catalytic amounts of low molecular weight carboxylic acids such as propionic acid or 2-ethylhexanoic acid as in U.S. Pat. No. 4,263,150 and U.S. Pat. No. 4,507,215. Typically these catalytic agents are used in as little as 0.01 equivalent or less per 1 phosphorus up to 0.1 or 0.2 or even 0.3 equivalents per 1 phosphorus of the dithiophosphoric acid or salt.

The term “hydrocarbyl,” when referring to groups attached to the remainder of a molecule, may refer to groups having a purely hydrocarbon or predominantly hydrocarbon character within the context of this invention. These groups include the following:

(1) Purely hydrocarbon groups; that is, aliphatic, alicyclic, aromatic, aliphatic- and alicyclic-substituted aromatic, aromatic-substituted aliphatic and alicyclic groups, and the like, as well as cyclic groups wherein the ring is completed through another portion of the molecule (that is, any two indicated substituents may together form an alicyclic group). Examples include methyl, octyl, cyclohexyl, phenyl, etc.

(2) Substituted hydrocarbon groups; that is, groups containing non-hydrocarbon substituents which do not alter the predominantly hydrocarbon character of the group. Examples include hydroxy, nitro, cyano, alkoxy, acyl, etc.

(3) Hetero groups; that is, groups which, while predominantly hydrocarbon in character, contain atoms other than carbon in a chain or ring otherwise composed of carbon atoms. Examples include nitrogen, oxygen and sulfur.

In general, no more than about three substituents or hetero atoms, and in one embodiment no more than one, may be present for each 10 carbon atoms in the hydrocarbyl group.

The term “lower” as used herein in conjunction with terms such as hydrocarbyl, alkyl, alkenyl, alkoxy, and the like, may describe such groups which contain a total of up to 7 carbon atoms.

The term “oil-soluble” may refer to a material that is soluble in mineral oil to the extent of at least about 0.5 gram per liter at 25° C.

The term “TBN” may refer to total base number. This is the amount of acid (perchloric or hydrochloric) needed to neutralize all or part of a material's basicity, expressed as milligrams of KOH per gram of sample.

The term “lean-phosphorus containing exhaust gas” may refer to an exhaust gas that is generated in an internal combustion engine lubricated with a lubricating oil composition containing an overbased low molecular weight zinc dialkyldithiophosphate with a metal ratio of at least about 1.08:1 or a Zn/P weight ratio of 1:1.15, the exhaust gas having a relatively low concentration of phosphorus when compared to an exhaust gas generated under the same conditions using the same lubricating oil composition containing the same level of phosphorus except that the phosphorus containing compound is a neutral low molecular weight zinc dialkyldithiophosphate or a neutral or overbased high-molecular weight zinc dialkyldithiophosphate.

The term “substantial absence of copper” may refer to the fact that copper may not be intentionally added to the lubricating oil composition and, if present, is present as an impurity. The concentration of this impurity may be no more than about 10 ppm, and in one embodiment no more than about 5 ppm, and in one embodiment no more than about 2 ppm. The concentration of copper may be in the range from about 0.1 to about 10 ppm, and in one embodiment in the range from about 0.1 to about 5 ppm, and in one embodiment in the range from about 0.1 to about 2 ppm.

The Lubricating Oil Composition.

The lubricating oil composition may comprise of one or more base oils which may be present in a major amount. The base oil may be present in an amount greater than about 60% by weight, and in one embodiment greater than about 70% by weight, and in one embodiment greater than about 80% by weight, and in one embodiment greater than about 85% by weight of the lubricating oil composition. The lubricating oil composition contains at least one overbased low molecular weight zinc dialkyldithiophosphates. The lubricating oil composition may contain an alkali or alkaline earth metal containing detergent, an acylated-nitrogen containing compound which may function as a dispersant, and/or at least one boron-containing compound. The lubricating oil composition may contain one or more other additives known in the art.

The lubricating oil composition may have a viscosity of up to about 16.3 mm²/sec at 100° C., and in one embodiment in the range from about 5 to about 16.3 mm²/sec at 100° C., and in one embodiment in the range from about 6 to about 13 mm²/sec at 100° C.

The lubricating oil composition may have an SAE Viscosity Grade of 0W, 0W-20, 0W-30, 0W-40, 0W-50, 0-60, 5W, 5W-20, 5W-30, 5W-40, 5W-50, 5W-60, 10W, 10W-20, 10W-30, 10W-40, 10W-50, 15W-40, 15W-50, 20W or 30W.

The lubricating oil composition may be characterized by a sulfur content of up to about 1% by weight, and in one embodiment up to about 0.5% by weight, and in one embodiment in the range from about 0.01% to about 1% by weight, and in one embodiment in the range from about 0.01% to about 0.5% by weight.

The lubricating oil composition may be characterized by a phosphorus content of up to about 0.12%, and in one embodiment up to about 0.10%, and in one embodiment up to about 0.08%, and in one embodiment up to about 0.05% by weight, and in one embodiment in the range from about 0.01 to about 0.12%, and in one embodiment in the range from about 0.01 to about 0.10%, and in one embodiment in the range from about 0.01 to about 0.08%, and in one embodiment in the range from about 0.01 to about 0.06%, and in one embodiment in the range form about 0.02 to about 0.12%, and in one embodiment in the range from about 0.02 to about 0.10%, and in one embodiment in the range from about 0.02 to about 0.08%, and in one embodiment in the range from about 0.02 to about 0.06%, and in one embodiment in the range from about 0.03 to about 0.12% by weight, and in one embodiment in the range from about 0.03 to about 0.10% by weight, and in one embodiment in the range from about 0.03 to about 0.08% by weight, and in one embodiment in the range from about 0.03 to about 0.06% by weight, and in one embodiment in the range from about 0.03% to about 0.05% by weight.

The lubricating oil composition may have a boron content in the range up to about 0.2% by weight, and in one embodiment in the range from about 0.01 to about 0.2% by weight, and in one embodiment in the range from about 0.02 to about 0.12% by weight, and in one embodiment in the range from about 0.05 to about 0.1% by weight.

The ash content of the lubricating oil composition as determined by the procedures in ASTM D-874-96 may be in the range from about 0.3 to about 1.4% by weight, and in one embodiment in the range from about 0.3 to about 1.2% by weight, and in one embodiment in the range from about 0.3 to about 1.0% by weight.

The lubricating oil composition may be characterized by a chlorine content of up to about 100 ppm, and in one embodiment up to about 50 ppm, and in one embodiment up to about 10 ppm.

The Base Oil

The base oil used in the lubricating oil composition may comprise any of the base oils in Groups I-V as specified in the American Petroleum Institute (API) Base Oil Interchangeability Guidelines. The five base oil groups are as follows:

Base Oil Viscosity Category Sulfur (%) Saturates (%) Index Group I   >0.03 and/or <90 80 to 120 Group II ≦0.03 and ≧90 80 to 120 Group III ≦0.03 and ≧90 ≧120 Group IV All polyalphaolefins (PAOs) Group V All others not included in Groups I, II, III or IV Groups I, II and III are mineral oil base stocks.

The base oil may be a natural oil, synthetic oil or mixture thereof. The natural oils include animal oils and vegetable oils (e.g., castor oil, lard oil) as well as mineral lubricating oils such as liquid petroleum oils and solvent treated or acid-treated mineral lubricating oils of the paraffinic, naphthenic or mixed paraffinic-naphthenic types. Oils derived from coal or shale are also useful.

Synthetic oils may include hydrocarbon oils such as polymerized and interpolymerized olefins, alkylbenzenes, polyphenyls, alkylated diphenyl ethers, alkylated diphenyl sulfides, and derivatives, analogs and homologs thereof. The synthetic oils include alkylene oxide polymers and interpolymers and derivatives thereof where the terminal hydroxyl groups have been modified by esterification, etherification, etc.; esters of dicarboxylic acids (e.g., phthalic acid, succinic acid, alkyl succinic acids, alkenyl succinic acids, etc.) with a variety of alcohols (e.g., butyl alcohol, hexyl alcohol, dodecyl alcohol, 2-ethylhexyl alcohol, ethylene glycol, etc.); and esters made from C₅ to C₁₂ monocarboxylic acids and polyols or polyol ethers.

In one embodiment, the base oil may comprise a polyalphaolefin (PAO), an oil derived from Fischer-Tropsch synthesized hydrocarbons, or an hydroisomerized Fischer-Tropsch hydrocarbon oil or wax. In one embodiment, Group II or Group III oils, or mixtures thereof may be used. In one embodiment, mixtures of Group III and Group IV oils may be used.

Unrefined, refined and rerefined oils, either natural or synthetic (as well as mixtures of two or more of any of these) of the type disclosed hereinabove may be used as the base oil.

The Overbased Low Molecular Weight Zinc Dialkyldithiophosphate

The overbased low molecular weight zinc dialkyldithiophosphate may be derived from one or more dithiophosphoric acids represented by the formula

and/or metal salt thereof wherein in Formula (I): R¹ and R² are independently hydrocarbyl groups. The average total number of carbon atoms in R¹ and R² in Formula I may be up to 10, and in one embodiment in the range from 4 to 10, and in one embodiment in the range from about 5 to 10, and in one embodiment in the range from about 6 to 10, and in one embodiment about 8. R¹ and R² may be independently hydrocarbyl groups of about 2 to about 8 carbon atoms, and in one embodiment 2 to about 7 carbon atoms, and in one embodiment about 2 to about 6 carbon atoms, and in one embodiment about 2 to about 5 carbon atoms, and in one embodiment about 4 carbon atoms. In one embodiment, from about 5 to about 70 mole % of the alkyl groups may be C₃ alkyl groups, and from about 70 to about 95 mole % of the groups may comprise one or more alkyl groups of from 2 to about 7 carbon atoms, and in one embodiment from about 2 to about 6 carbon atoms.

R¹ and R² may be independently alkyl groups, alkenyl groups, or mixtures thereof. R¹ and R² may be derived from one or more primary alcohols, one or more secondary alcohols, or a mixture of at least one primary alcohol and at least one secondary alcohol. Examples of R¹ and R² may include ethyl, isopropyl, butyl, isobutyl, pentyl, 1,3 dimethyl-butyl, isooctyl, and the like. In one embodiment, the low molecular weight dialkyldithiophosphoric acid may be derived from a C₄ alcohol.

The metal ratio of the overbased low molecular weight zinc dialkyldithiophosphate may be at least about 1.15:1, and in one embodiment in the range from about 1.15:1 to about 1.5:1, and in one embodiment in the range from about 1.2:1 to about 1.4:1. A metal ratio in the range from 1:1 to 1.07:1 or a Zn/P wt ratio of up to about 1.12 may be referred to as being neutral or substantially neutral.

The overbased low molecular zinc dialkyldithiophosphate may be employed in the lubricating oil composition at a concentration sufficient to provide the lubricating oil composition with a phosphorus concentration in the range up to about 0.12% by weight, and in one embodiment in the range from about 0.03 to about 0.12% percent by weight, and in one embodiment in the range from about 0.03% to about 0.10% by weight, and in one embodiment in the range from about 0.03 to about 0.08% by weight, and in one embodiment in the range from about 0.03 to about 0.05% by weight.

The Acylated Nitrogen Containing Compound

The acylated nitrogen containing compound may contain a substituent comprising at least about 30 aliphatic carbon atoms and may be made by reacting at least one carboxylic acid acylating agent with at least one amino compound. The acylating agent may be linked to the amino compound through an imido, amido, amidine or salt linkage. The substituent comprising at least about 30 aliphatic carbon atoms may be in either the carboxylic acid acylating agent derived portion of the molecule or in the amino compound derived portion of the molecule.

These substituents may be hydrocarbyl groups made from homo- or interpolymers (e.g., copolymers, terpolymers) of mono- or di-olefins having 2 to about 10 carbon atoms, such as ethylene, propylene, 1-butene, isobutene, butadiene, isoprene, 1-hexene, 1-octene, etc. Typically, these olefins are 1-monoolefins. The substituent may also be derived from the halogenated (e.g., chlorinated or brominated) analogs of such homo- or interpolymers.

A useful source for the substituent groups may comprise poly(isobutene)s obtained by the polymerization of a C₄ refinery stream having a butene content of about 35 to about 75 weight percent and an isobutene content of about 30 to about 60 weight percent in the presence of a Lewis acid catalyst such as aluminum trichloride or boron trifluoride. These polybutenes may contain predominantly isobutene repeating units.

The substituent may comprise a polyisobutene group derived from a polyisobutene having a high methylvinylidene isomer content, that is, at least about 50% methylvinylidene, and in one embodiment at least about 70% methylvinylidene. These high methylvinylidene polyisobutenes may include those prepared using boron trifluoride catalysts.

The number average molecular weight of the acylating agent may vary from about 300 up to about 5,000, 10,000 or 20,000. In one embodiment, the acylating agent may be a hydrocarbyl substituted succinic acid or anhydride containing hydrocarbyl substituent groups and succinic groups wherein the substituent groups are derived from a polyalkene such as polyisobutene. The acid or anhydride may be characterized by the presence within its structure of an average of at least about 1 succinic group for each equivalent weight of substituent groups, and in one embodiment from about 1 to about 2.5 succinic groups for each equivalent weight of substituent groups. The polyalkene may have number average molecular weight ( M _(n)) of at least about 700, and in one embodiment about 700 to about 3000, and in one embodiment about 900 to about 2200. The ratio between the weight average molecular weight ( Mw) and the ( Mn) (that is, Mw/ Mn) may range from about 1 to about 10, and in one embodiment about 1.5 to about 5, and in one embodiment about 2.5 to about 5. The number of equivalent weights of substituent groups may be deemed to be the number corresponding to the quotient obtained by dividing the Mn value of the polyalkene from which the substituent is derived into the total weight of the substituent groups present in the substituted succinic acid or anhydride.

The amino compound may be characterized by the presence within its structure of at least one HN<group and may be a monoamine or polyamine. Mixtures of two or more amino compounds may be used in the reaction with one or more acylating reagents. In one embodiment, the amino compound may contain at least one primary amino group (i.e., —NH₂). In one embodiment, the amine may be a polyamine, for example, a polyamine containing at least two —NH— groups, either or both of which are primary or secondary amines. The amines may be aliphatic, cycloaliphatic, aromatic or heterocyclic amines. Hydroxy substituted amines, such as alkanol amines (e.g., mono- or diethanol amine), and hydroxy (polyhydrocarbyloxy) anologs of such alkanol amines may be used.

Among the useful amines are the alkylene polyamines, including the polyalkylene polyamines. The alkylene polyamines may include those represented by the formula

wherein in Formula (II), n is from 1 to about 14; each R is independently a hydrogen atom, a hydrocarbyl group or a hydroxy-substituted or amine-substituted hydrocarbyl group having up to about 30 atoms, or two R groups on different nitrogen atoms can be joined together to form a U group, with the proviso that at least one R group is a hydrogen atom and U is an alkylene group of about 2 to about 10 carbon atoms. U may be ethylene or propylene. Alkylene polyamines where each R is hydrogen or an amino-substituted hydrocarbyl group with the ethylene polyamines and mixtures of ethylene polyamines are useful. Usually n will have an average value of from about 2 to about 10. Such alkylene polyamines include methylene polyamines, ethylene polyamines, propylene polyamines, butylene polyamines, pentylene polyamines, hexylene polyamines, heptylene polyamines, amino propylated ethylene polyamines, etc. The higher homologs of such amines and related amino alkyl-substituted piperazines may be included.

Alkylene polyamines that may be useful may include ethylene diamine, diethylene triamine, triethylene tetramine, tetraethylene pentamine, pentaethylene hexamine, propylene diamine, trimethylene diamine, hexamethylene diamine, decamethylene diamine, octamethylene diamine, di(heptamethylene) triamine, tripropylene tetramine, trimethylene diamine, di(trimethylene)triamine, N-(2-amino-ethyl)piperazine, 1,4-bis(2-aminoethyl)piperazine, and the like. Higher homologs such as those obtained by condensing two or more of the above-illustrated alkylene amines may be used. Mixtures of two or more of any of the afore-described polyamines may be used.

Useful polyamines may include those resulting from stripping polyamine mixtures. In this instance, lower molecular weight polyamines and volatile contaminants are removed from an alkylene polyamine mixture to leave as residue what is often termed “polyamine bottoms”. In general, alkylene polyamine bottoms can be characterized as having less than about 2% by weight, and in one embodiment less than about 1% by weight material boiling below about 200° C.

The acylated nitrogen containing compounds may include amine salts, amides, imides, amidines, amidic acids, amidic salts and imidazolines as well as mixtures thereof. To prepare the acylated nitrogen-containing compounds from the acylating agents and the amino compounds, one or more acylating reagents and one or more amino compounds may be heated, optionally in the presence of a normally liquid, substantially inert organic liquid solvent/diluent, at temperatures in the range of 80° C. up to the decomposition point of any of the reactants or the product but normally at temperatures in the range of about 100° C. to about 300° C., provided 300° C. does not exceed the decomposition point of any of the reactants or the product. Temperatures of about 125° C. to about 250° C. may be used. The acylating agent and the amino compound may be reacted in amounts sufficient to provide from about 0.5 to about 3 moles of amino compound per equivalent of acylating agent. The number of equivalents of the acylating agent may vary with the number of carboxy groups present therein. In determining the number of equivalents of the acylating agent, those carboxyl functions which are not capable of reacting as a carboxylic acid acylating agent are excluded. In general, however, there is one equivalent of acylating agent for each carboxy group in the acylating agent.

The use of acylated nitrogen containing compounds with relatively high TBNs in the lubricating oil composition may tend to reduce the volatility of the phosphorus in the overbased low molecular weight zinc dialkyldithiophosphate. Accordingly, in one embodiment, the acylated nitrogen containing compound may have a TBN (on an oil-free basis) of at least about 2, and in one embodiment in the range from about 2 to about 60, and in one embodiment in the range from about 5 to about 30, and in one embodiment in the range from about 10 to about 20.

The acylated nitrogen containing compound may be employed in the lubricating oil composition at a concentration in the range from about 1% to about 20% by weight, and in one embodiment in the range from about 1% to about 10% percent by weight, and in one embodiment in the range from about 1% to about 5% by weight.

The Alkali or Alkaline Earth Metal Containing Detergent

The alkali metal or alkaline earth metal containing detergent may be an alkali or alkaline earth metal salt of an acidic organic compound. The acidic organic compound may be an organic sulfur acid, phenol, carboxylic acid or derivative thereof. The acidic organic compound may be a salixarate. These salts may be neutral or overbased. The former contain an amount of metal cation just sufficient to neutralize the acidic groups present in the salt anion; the latter contain an excess of metal cation and are often termed basic, overbased, hyperbased or superbased salts. These salts may have a TBN in the range from about 30 to about 460, and in one embodiment in the range from about 100 to about 400, and in one embodiment in the range from about 200 to about 400, and in one embodiment in the range from about 300 to about 400.

The organic sulfur acids may be oil-soluble organic sulfur acids such as sulfonic, sulfamic, thiosulfonic, sulfinic, sulfenic, partial ester sulfuric, sulfurous and thiosulfuric acid. Generally they are salts of aliphatic or aromatic sulfonic acids. The sulfonic acids include the mono- or poly-nuclear aromatic or cycloaliphatic compounds.

The carboxylic acids may include aliphatic, cycloaliphatic, and aromatic mono- and polybasic carboxylic acids such as the naphthenic acids, alkyl- or alkenyl-substituted cyclopentanoic acids, alkyl- or alkenyl-substituted cyclohexanoic acids, alkyl- or alkenyl-substituted aromatic carboxylic acids. The aliphatic acids may contain at least about 8 carbon atoms, and in one embodiment at least about 12 carbon atoms. Usually they have no more than about 400 carbon atoms. The cycloaliphatic and aliphatic carboxylic acids may be saturated or unsaturated.

A useful group of carboxylic acids may be the oil-soluble aromatic carboxylic acids. These acids may be represented by the formula:

(R*)_(a)—Ar*(CXXH)_(m)  (III)

wherein in Formula (III), R* is an aliphatic hydrocarbyl group of about 4 to about 400 carbon atoms, a is an integer of from one to four, Ar* is a polyvalent aromatic hydrocarbon nucleus of up to about 14 carbon atoms, each X is independently a sulfur or oxygen atom, and m is an integer of from one to four with the proviso that R* and a are such that there is an average of at least about 8 aliphatic carbon atoms provided by the R* groups for each acid molecule.

A useful group of carboxylic acids may be the aliphatic-hydrocarbon substituted salicylic acids wherein each aliphatic hydrocarbon substituent contains an average of at least about 8 carbon atoms, and in one embodiment at least about 16 carbon atoms per substituent, and the acids contain one to three substituents per molecule. A useful aliphatic-hydrocarbon substituted salicylic acid is C₁₆-C₁₈ alkyl salicylic acid. A group of carboxylic acid derivatives that are useful are the lactones represented by the formula

wherein in Formula (IV), R¹, R², R³, R⁴, R⁵ and R⁶ are independently H, hydrocarbyl groups or hydroxy substituted hydrocarbyl groups of from 1 to about 30 carbon atoms, with the proviso that the total number of carbon atoms must be sufficient to render the lactones oil soluble; R² and R³ can be linked together to form an aliphatic or aromatic ring; and a is a number in the range of zero to 4. A useful lactone can be prepared by reacting an alkyl (e.g., dodecyl)phenol with glyoxylic acid at a molar ratio of about 2:1.

Neutral and basic salts of phenols (generally known as phenates) are also useful in the compositions of this invention and well known to those skilled in the art. The phenols from which these phenates are formed are of the general formula

(R*)_(a)—(Ar*)-(OH)_(m)  (V)

wherein in Formula (V), R*, a, Ar*, and m have the same meaning as described hereinabove with reference to Formula (III).

The salixarate may be a substantially linear compound comprising at least one unit of formula (VI) or formula (VII):

each end of the compound having a terminal group of formula (VIII) or formula (IX):

such groups being linked by divalent bridging groups A, which may be the same or different for each linkage; wherein in formulas (VI)-(X) R³ is hydrogen or a hydrocarbyl group; R² is hydroxyl or a hydrocarbyl group and j is 0, 1, or 2; R⁶ is hydrogen, a hydrocarbyl group, or a hetero-substituted hydrocarbyl group; either R⁴ is hydroxyl and R⁵ and R⁷ are independently either hydrogen, a hydrocarbyl group, or hetero-substituted hydrocarbyl group, or else R⁵ and R⁷ are both hydroxyl and R⁴ is hydrogen, a hydrocarbyl group, or a hetero-substituted hydrocarbyl group; provided that at least one of R⁴, R⁵, R⁶ and R⁷ is hydrocarbyl containing at least 8 carbon atoms; and wherein the molecules on average contain at least one of unit (VI) or (VIII) and at least one of unit (VII) or (IX) and the ratio of the total number of units (VI) and (VIII) to the total number of units of (VII) and (IX) in the composition is about 0.1:1 to about 2:1.

The divalent bridging group “A,” which may be the same or different in each occurrence, includes —CH₂— (methylene bridge) and —CH₂OCH₂— (ether bridge), either of which may be derived from formaldehyde or a formaldehyde equivalent (e.g., paraform, formalin).

Salixarate derivatives and methods of their preparation are described in greater detail in U.S. Pat. No. 6,200,936 and PCT Publication WO 01/56968. It is believed that the salixarate derivatives have a predominantly linear, rather than macrocyclic, structure, although both structures are intended to be encompassed by the term “salixarate.”

Mixtures of two or more neutral or basic metal salts of the hereinabove described acidic organic compounds may be used in the lubricating oil compositions.

The alkali and alkaline earth metals that may be useful may include sodium, potassium, lithium, calcium, magnesium, strontium and barium. Sodium, lithium and calcium may be especially useful.

The use of sodium in the lubricating oil composition may tend to decrease the volatility of the phosphorus used therein significantly. Accordingly, in one embodiment of the invention, the use of sodium as the detergent metal may be particularly useful.

The alkali or alkaline earth metal containing detergent may be employed in the lubricating oil composition at a concentration in the range from about 0.1 to about 10% by weight, and in one embodiment in the range from about 0.2 to about 5% percent by weight, and in one embodiment in the range from about 0.3% to about 3% by weight, and in one embodiment in the range from about 0.5 to about 2% by weight.

The Boron-Containing Compound

The boron-containing compound may be is a compound represented by one or more of the formulae

wherein in Formulae XI-XIII, each R may be independently an organic group and any two adjacent R groups may together form a cyclic group. Mixtures of two or more of the foregoing may be used. In one embodiment, each R may be independently a hydrocarbyl group. The total number of carbon atoms in the R groups in each formula may be sufficient to render the compound soluble in the base oil. Generally, the total number of carbon atoms in the R groups may be at least about 8, and in one embodiment at least about 10, and in one embodiment at least about 12. There may be no limit to the total number of carbon atoms in the R groups that is required, but a practical upper limit may be about 400 or about 500 carbon atoms. In one embodiment, each R group may be independently a hydrocarbyl group of 1 to about 100 carbon atoms, and in one embodiment 1 to about 50 carbon atoms, and in one embodiment 1 to about 30 carbon atoms, and in one embodiment 1 to about 10 carbon atoms, with the proviso that the total number of carbons in the R group may be at least about 8. Each R group may be the same as the other, although they may be different. Examples of useful R groups may include isopropyl, n-butyl, isobutyl, amyl, 1,3 dimethyl-butyl, 2-ethyl-1-hexyl, isooctyl, decyl, dodecyl, tetradecyl, 2-pentenyl, dodecenyl, phenyl, naphthyl, alkylphenyl, alkylnaphthyl, phenylalkyl, naphthylalkyl, alkylphenylalkyl, alkylnaphthylalkyl, and the like.

In one embodiment, the boron-containing compound may be a compound represented by the formula

wherein in Formula (XIV): R¹, R², R³ and R⁴ are independently hydrocarbyl groups of 1 to about 12 carbon atoms; and R⁵ and R⁶ are independently alkylene groups of 1 to about 6 carbon atoms, and in one embodiment about 2 to about 4 carbon atoms, and in one embodiment about 2 or about 3 carbon atoms. In one embodiment, R¹ and R² may independently contain 1 to about 6 carbon atoms, and in one embodiment each may be a t-butyl group. In one embodiment, R³ and R⁴ are independently hydrocarbyl groups of about 2 to about 12 carbon atoms, and in one embodiment about 8 to about 10 carbon atoms. In one embodiment, R⁵ and R⁶ are independently —CH₂CH₂— or —CH₂CH₂CH₂—.

In one embodiment, the boron-containing compound may be a compound represented by the formula:

wherein in Formula (XV): R¹, R², R³, R⁴, R⁵, R⁶, R⁷ and R⁸ are independently hydrogen or hydrocarbyl groups. Each of the hydrocarbyl groups may contain from 1 to about 12 carbon atoms, and in one embodiment 1 to about 4 carbon atoms. An example is 2,2′-oxy-bis-(4,4,6-trimethyl-1,3,2-dioxaborinane).

A useful boron-containing compound may be available from Crompton Corporation under the trade designation LA-2607. This material may be identified as a phenolic borate having the structure represented by Formula (XIV) wherein R¹ and R² are each t-butyl, R³ and R⁴ are hydrocarbyl groups of 2 to about 12 carbon atoms, R⁵ is —CH₂CH₂—, and R⁶ is —CH₂CH₂CH₂—.

In one embodiment, the boron-containing compound may be a compound represented by the formula B(OC₅H₁₁) or B(OC₄H₉)₃. A useful boron-containing compound may be available from Mobil under the trade designation MCP-1286; this material may be identified as a borated ester.

The boron-containing compound may be employed in the lubricating oil composition at a sufficient concentration to provide the lubricating oil composition with a boron concentration in the range up to about 0.2% by weight, and in one embodiment in the range from about 0.01% to about 0.2% by weight, and in one embodiment in the range from about 0.02% to about 0.12% by weight, and in one embodiment in the range from about 0.05% to about 0.1% by weight. These compounds may be added directly to the lubricating oil composition. In one embodiment, however, they may be diluted with a substantially inert, normally liquid organic diluent such as mineral oil, synthetic oil (e.g., ester of dicarboxylic acid), naptha, alkylated (e.g. C₁₀-C₁₃ alkyl)benzene, toluene or xylene to form an additive concentrate. These concentrates may contain from about 1% to about 99% by weight, and in one embodiment about 10% to about 90% by weight of the diluent.

Additional Lubricating Oil Additives

The lubricating oil composition may also contain other lubricant additives. These may include, for example, corrosion-inhibiting agents, antioxidants, viscosity modifiers, dispersant viscosity index modifiers, pour point depressants, friction modifiers, antiwear agents other than those discussed above, EP agents other than those discussed above, dispersants other than those discussed above, detergents other than those discussed above, fluidity modifiers, copper passivators, anti-foam agents, etc. Each of the foregoing additives, when used, may be used at a functionally effective amount to impart the desired properties to the lubricant. Generally, the concentration of each of these additives, when used, may be in the range from about 0.001% to about 20% by weight, and in one embodiment in the range from about 0.01% to about 10% by weight based on the total weight of the lubricating oil composition.

Concentrates and Diluents

The foregoing lubricating oil additives may be added directly to the base oil to form the lubricating oil composition. In one embodiment, however, one or more of the additives may be diluted with a substantially inert, normally liquid organic diluent such as mineral oil, synthetic oil, naphtha, alkylated (e.g., C₁₀-C₁₃ alkyl)benzene, toluene or xylene to form an additive concentrate. These concentrates may contain from about 1% to about 99% by weight, and in one embodiment from about 10% to 90% by weight of such diluent. The concentrates may be added to the base oil to form the lubricating oil composition.

The Method of Lubricating the Engine and Improving Efficiency of the Emissions Control System

The disclosed method may provide for lubricating an internal combustion engine and improving the efficiency of the emissions control system of the engine wherein the emissions control system is equipped with a catalyst-containing exhaust gas after treatment device used with the engine. The lubricating oil composition may generate a lean-phosphorus containing exhaust gas during operation of the engine. The lean-phosphorus containing exhaust gas may be advanced to the exhaust gas after treatment device. In the exhaust gas after treatment device the lean-phosphorus containing exhaust gas may contact the catalyst. The phosphorus in the lean-phosphorus containing exhaust gas may contaminate the catalyst and thereby reduce its efficiency. However, since the level of phosphorus in the lean-phosphorus containing exhaust gas may be at a reduced level, the amount of contamination of the catalyst may be reduced. This reduction in contamination may result in an improvement in the efficiency of the exhaust gas after treatment device.

The generation of a lean-phosphorus containing exhaust gas may be dependent on the use of the lubricating oil composition. The use of the overbased low molecular weight dialkyldithiophosphate in the lubricating oil composition may provide for low-phosphorus volatility when the lubricating oil composition is used to lubricate an internal combustion engine. As indicated above, the lubricating oil composition may also contain one or more alkali or alkaline earth metal containing detergents, one or more acylated-nitrogen containing compounds, and/or one or more boron containing compounds. The combination of these additives with the overbased low molecular weight dialkyldithiophosphate may contribute to reducing the volatility of the overbased low molecular weight zinc dialkyldithiophosphate. Additional optional nitrogen-containing compounds (e.g., antioxidants) when present may further contribute to this effect. This reduction in phosphorus volatility may provide for the generation of a lean-phosphorus containing exhaust gas. In one embodiment, the weight ratio of detergent metal to phosphorus in the lubricating oil composition at the time the lubricating oil composition may be added to the engine may be in the range from about 0.5:1 to about 10:1, and in one embodiment in the range from about 2:1 to about 4:1, and in one embodiment in the range from about 2.5:1 to about 3:1. In one embodiment, the weight ratio of basic (titratable) nitrogen to phosphorus in the lubricating oil composition at the time the lubricating oil composition may be added to the engine may be in the range from about 0.3:1 to about 4:1, and in one embodiment about 0.5:1 to about 2:1, and in one embodiment in the range from about 1:1 to about 1.5:1.

The amount of phosphorus in the exhaust gas during the operation of the engine may be indirectly proportional to the amount of phosphorus retained in the lubricating oil composition in the crankcase. The amount of phosphorus retained in the crankcase may be calculated from the following formula:

${\% \mspace{14mu} P_{retention}} = {\frac{\left( {\% \mspace{14mu} {wt}\mspace{14mu} P_{drain}} \right)\left( {\% \mspace{14mu} {wt}\mspace{14mu} M_{new}} \right)}{\left( {\% \mspace{14mu} {wt}\mspace{14mu} P_{new}} \right)\left( {\% \mspace{14mu} {wt}\mspace{14mu} M_{drain}} \right)} \times 100}$

wherein: % wt P_(drain) is the percent by weight of phosphorus in the lubricating oil composition in the crankcase at the end of a drain interval; % wt M_(new) is the percent by weight of detergent metal in the lubricating oil composition in the crankcase at the beginning of the drain interval; % wt P_(new) is the percent by weight of phosphorus in the lubricating oil composition in the crankcase at the beginning of the drain interval; and % M_(drain) is the percent by weight of detergent metal in the lubricating oil composition at the end of the drain interval. The amount of phosphorus retained in the crankcase oil of the engine after a 12000 kilometer (7500 mile) drain cycle may be at least about 80% by weight, and in one embodiment at least about 84% by weight, and in one embodiment at least about 88% by weight, and in one embodiment at least about 92% by weight, and in one embodiment at least about 95% by weight, and in one embodiment at least about 98% by weight. The amount of phosphorus lost from the crankcase oil with the exhaust gas over a 7500 mile (12000 kilometer) drain cycle may be about 20% by weight or less, and in one embodiment about 16% by weight or less, and in one embodiment about 12% by weight or less, and in one embodiment about 8% by weight or less, and in one embodiment about 5% by weight or less, and in one embodiment about 2% by weight or less.

Copper in the lubricating oil composition may increase the volatility of the phosphorus used therein. Accordingly, at the time of its addition to the engine, the lubricating oil composition may be characterized by the substantial absence of copper.

The internal combustion engine may be any internal combustion engine that is equipped with exhaust gas after treatment device that utilizes a catalyst. These may include engines that employ a closed crankcase system and positive crankcase ventilation. The internal combustion engine may be a spark-ignited or a compression-ignited engine. These engines may include automobile and truck engines, two-cycle engines, aviation piston engines, marine and railroad diesel engines, and the like. These may include on- and off-highway engines. The compression-ignited engines may include those for both mobile and stationary power plants. The compression-ignited engines may include those used in urban buses, as well as all classes of trucks. The compression-ignited engines may be of the two-stroke per cycle or four-stroke per cycle type. The compression-ignited engines may include heavy duty diesel engines.

The exhaust gas after treatment device may be referred to as a catalytic converter and may be of any conventional design. The exhaust gas after treatment device may comprise flow-through passages of ceramic or metal coated with a washcoat comprising zeolite, Al₂O₃, SiO₂, TiO₂, CeO₂, ZrO₂, V₂O₅, La₂O₃, or mixtures of two or more thereof. The washcoat may support a catalyst comprising Pt, Pd, Rh, Ir, Ru, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ag, Ce, Ga, or a mixture of two or more thereof.

EXAMPLES

Engine tests are conducted using a 1997 GM 5.2 L V-8 DOHC equipped with a roller-follower valve train. The test duration is 25 hours. The engine conditions are: 2640 rpm; 29.8 kilowatt load (calculated); 210° F. (98.9° C.) oil temperature; and 180° F. (82.2° C.) coolant temperature. The exhaust gas is analyzed for volatile phosphorus compounds. Elemental analysis (by ICP) and phosphorus NMR are used. The lubricating oil compositions used in the tests are provided in Table I. In Table I, unless otherwise indicated, all numerical values are in parts by weight.

TABLE I Ingredient Parts by Wt Base Oil: Group II, to make 5W-30 oil 100 Viscosity Modifier: LZ7070D available from 6.5 Lubrizol identified as olefin copolymer dispersed in oil (91% diluent oil) Pour point depressant: esterified styrene- 0.3 maleic anhydride copolymer dispersed in oil (53.6% diluent oil) Ashless organic friction modifier 0.5 Dispersant: succinimide derived from 5.1 polyisobutene (Mn = 2000) substituted succinic anhydride and polyethylene amines dispersed in oil, TBN = 16 (56% diluent oil) Antioxidant: nonylated diphenylamine 0.7 Antioxidant: sulfurized olefin containing 0.2 13.9% sulfur dispersed with oil (5% diluent oil) Hindered phenolic ester antioxidant 0.2 Detergent: calcium sulfonate dispersed in oil, 0.88 TBN = 300 (42% diluent oil) Detergent: calcium sulfonate dispersed in oil, 0.65 TBN = 400 (42% diluent oil) ZDDPs described in Table II (% P) 0.075

The results of these engine tests are provided in Table II. In Table II, the term “Blow-by Phosphorus” refers to phosphorus in the engine exhaust that contacts the catalyst in the engine's emission control system. These results indicate a significant reduction in phosphorus volatility when overbased low molecular weight ZDDPs are used (Examples 1 and 2) as compared to a substantially neutral low molecular weight ZDDP (Example C-1), a substantially neutral high molecular weight ZDDP (Example C-2) and overbased high molecular weight ZDDP (Example C-3).

TABLE II Blow-By Example ZDDP Description Phosphorus 1 ZDDP derived from C_(4.2 avg) ROH with 418 Zn:P wt ratio of = 1.16:1 2 ZDDP derived from C_(4.2 avg) ROH with 412 Zn:P wt ratio = = 1.32:1 C-1 ZDDP derived from C_(4.2 avg) ROH with 481 wt Zn:P ratio = 1.11:1 C-2 ZDDP derived from C₈ ROH with Zn:P 348 wt ratio = 1.11:1 C-3 ZDDP derived from C₈ ROH with Zn:P 354 wt ratio >1.2:1

The results of Examples 1 and 2 are almost as good as Examples C-2 and C-3 in terms of blow-by phosphorus, in spite of their lower molecular weight substituents. The lower molecular weight substituents are believed to provide improved wear protection as compared with the longer chain materials of Examples C-2 and C-3.

While the disclosed technology has been explained in relation to various embodiments, it is to be understood that various modifications thereof may become apparent to those skilled in the art upon reading the specification. Therefore, it is to be understood that the invention disclosed herein is intended to cover such modifications as fall within the scope of the appended claims. 

1. A lubricating oil composition comprising at least one base oil and at least one overbased low molecular weight zinc dialkyldithiophosphate derived from one or more dithiophosphoric acids represented by the formula

and/or metal salt thereof, wherein in formula (I), R¹ and R² are independently hydrocarbyl groups, the average total number of carbon atoms in R¹ and R² in formula (I) being in the range from 4 to 10, the metal ratio of the overbased low molecular weight zinc dialkyldithiophosphate being at least about 1.08:1, the lubricating oil composition being characterized by a phosphorus concentration of up to about 0.12% by weight.
 2. The composition of claim 1 wherein the lubricating oil composition has a viscosity of up to about 16.3 mm²/sec at 100° C.
 3. The composition of claim 1 wherein the lubricating oil composition has an SAE Viscosity Grade of 0W, 0W-20, 0W-30, 0W-40, 0W-50, 0W-60, 5W, 5W-20, 5W-30, 5W-40, 5W-50, 5W-60, 10W, 10W-20, 10W-30, 10W-40, 10W-50, 15W-40, 15W-50, 20W or 30W.
 4. The composition of claim 1 wherein the base oil comprises mineral oil.
 5. The composition of claim 1 wherein the base oil comprises a poly-alpha-olefin, an oil derived from Fischer-Tropsch synthesized hydrocarbons, or an hydroisomerized Fischer-Tropsch hydrocarbon oil or wax.
 6. The composition of claim 1 wherein in formula (I), R¹ and R² are independently alkyl groups of 2 to about 5 carbon atoms.
 7. The composition of claim 1 wherein in formula (I), R¹ and R² are each alkyl groups of about 4 carbon atoms.
 8. The composition of claim 1 wherein the metal ratio of the overbased zinc dialkyldithiophosphate is in the range from about 1.15 to about 1.5.
 9. The composition of claim 1 wherein the lubricating oil composition further comprises at least one alkali or alkaline earth metal containing detergent.
 10. The composition of claim 1 wherein the lubricating oil composition further comprises at least one acylated nitrogen containing compound having at least about 30 aliphatic carbon atoms.
 11. The composition of claim 9 wherein the alkali or alkaline earth metal-containing detergent comprises at least one salt of at least one organic sulfur acid, carboxylic acid, lactone, phenol, salixarate, or mixture of two or more thereof.
 12. The composition of claim 9 wherein the alkali or alkaline earth metal-containing detergent comprises at least one salt of at least one salixarate.
 13. The composition of claim 9 wherein the alkali or alkaline earth metal comprises sodium, lithium, calcium, or a mixture of two or more thereof.
 14. The composition of claim 10 wherein the acylated nitrogen-containing compound is derived from at least one carboxylic acylating agent and at least one amino compound containing at least one —NH— group, the acylating agent being linked to the amino compound through an imido, amido, amidine or salt linkage.
 15. The composition of claim 14 wherein the carboxylic acylating agent is a mono- or polycarboxylic acid or anhydride containing an aliphatic hydrocarbyl substituent of at least about 30 carbon atoms.
 16. The composition of claim 14 wherein the amino compound is an alkylenepolyamine represented by the formula:

wherein U is an alkylene group of from 2 to about 10 carbon atoms; each R is independently a hydrogen atom, a hydrocarbyl group, a hydroxy-substituted hydrocarbyl group, or an amine-substituted hydrocarbyl group containing up to about 30 carbon atoms; and n is 1 to about
 14. 17. The composition of claim 10 wherein the acylated nitrogen containing compound comprises at least one polyisobutene substituted succinimide.
 18. The composition of claim 1 wherein the composition further comprises at least one boron-containing compound represented by the formula

wherein in Formulae XI, XII and XIII each R is independently an organic group and any two adjacent R groups may together form a cyclic group.
 19. The composition of claim 1 wherein the lubricating oil composition further comprises one or more: dispersants, corrosion-inhibiting agents, antioxidants, viscosity modifiers, dispersant viscosity index modifiers, pour point depressants, friction modifiers, anti-wear agents, extreme pressure agents, fluidity modifiers, copper passivators, anti-foam agents, or a mixture of two or more thereof.
 20. A lubricating oil composition comprising at least one base oil, at least one overbased low molecular weight zinc dialkyldithiophosphate derived from one or more dithiophosphoric acids represented by the formula

and/or a metal salt thereof, wherein in formula (I), R¹ and R² are independently hydrocarbyl groups, the average total number of carbon atoms in R¹ and R² in formula (I) being in the range from about 6 to 10, the metal ratio of the overbased low molecular weight dialkyldithiophosphate being at least about 1.08:1, at least one alkali or alkaline earth metal detergent, the weight ratio of detergent metal to phosphorus in the lubricating oil composition being in the range from about 0.5:1 to about 10:1, and at least one nitrogen containing compound having at least about 30 aliphatic carbon atoms and a TBN of at least about 2, the weight ratio of nitrogen to phosphorus in the lubricating oil composition being in the range from about 0.3:1 to about 4:1, the lubricating oil composition being characterized by a phosphorus concentration of up to about 0.12% by weight.
 21. A method of lubricating an internal combustion engine and improving the efficiency of the emissions control system of the engine, the emissions control system being equipped with a catalyst containing exhaust gas after treatment device, the method comprising: (A) lubricating the engine with a lubricating oil composition comprising at least one base oil and at least one overbased low molecular weight zinc dialkyldithiophosphate derived from one or more dithiophosphoric acids represented by the formula

and/or metal salt thereof, wherein in formula (I), R¹ and R² are independently hydrocarbyl groups, the average total number of carbon atoms in R¹ and R² in formula (I) being in the range from 4 to 10, the metal ratio of the overbased low molecular weight dialkyldithiophosphate being at least about 1.08:1, the lubricating oil composition being characterized by a phosphorus concentration of up to about 0.12% by weight; (B) generating a lean-phosphorus containing exhaust gas; and (C) contacting the catalyst in the exhaust gas after treatment device with the lean-phosphorus containing exhaust gas. 